Update API for working tests
Fixes for final APE5 compatibility and other last-minute tests
minor tweak to facilitate subclassing behavior
Fix coordinates Sphinx build
Address comments by @eteq for coordinates docstrings
Fix for lingering doc issues in coordinates
Added changelog and backwards-incompatibility messages for coordinates
Remove warning from top of coordinates docs
Add missing SkyCoord.__nonzero__ and associated tests

closes #2005
Representation classes now only accept Quantity to simplify API and behavior of low-level frame classes
Updated API tests
Provide backward-compatibility layer in frames
Fix backward-compatibility layer
Make frame / SkyCoord transform_to accept SkyCoord object
Add back APE5 tests but simply update them
Disallow non-Quantity input for CartesianRepresentation
Move up deprecation message
Fix test failures resulting from changes to CartesianRepresentation
fix failing doctests
Updated message about string initialization deprecation to be more verbose